/Engine/Source/Runtime/StaticMeshDescription/Public/StaticMeshOperations.h
Return the number of unique vertices, unique vertices are the result of welding all similar vertex instances (position, UV, tangent space, color,...)
static int32 GetUniqueVertexCount ( const [FMeshDescription](API\Runtime\MeshDescription\FMeshDescription) & MeshDescription, const [FOverlappingCorners](API\Runtime\MeshUtilitiesCommon\FOverlappingCorners) & OverlappingCorners )